Re: [PATCH v4 bpf-next 1/5] bpf: add in-kernel split BTF support

[Song Liu <songliubraving@fb.com>]

> On Nov 10, 2020, at 10:31 AM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Tue, Nov 10, 2020 at 9:50 AM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Nov 9, 2020, at 5:19 PM, Andrii Nakryiko <andrii@kernel.org> wrote:
>>> 
>>> Adjust in-kernel BTF implementation to support a split BTF mode of operation.
>>> Changes are mostly mirroring libbpf split BTF changes, with the exception of
>>> start_id being 0 for in-kernel implementation due to simpler read-only mode.
>>> 
>>> Otherwise, for split BTF logic, most of the logic of jumping to base BTF,
>>> where necessary, is encapsulated in few helper functions. Type numbering and
>>> string offset in a split BTF are logically continuing where base BTF ends, so
>>> most of the high-level logic is kept without changes.
>>> 
>>> Type verification and size resolution is only doing an added resolution of new
>>> split BTF types and relies on already cached size and type resolution results
>>> in the base BTF.
>>> 
>>> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>>> ---
>>> kernel/bpf/btf.c | 171 +++++++++++++++++++++++++++++++++--------------
>>> 1 file changed, 119 insertions(+), 52 deletions(-)
>>> 
>>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>>> index 6324de8c59f7..727c1c27053f 100644
>>> --- a/kernel/bpf/btf.c
>>> +++ b/kernel/bpf/btf.c
>>> @@ -203,12 +203,17 @@ struct btf {
>>>      const char *strings;
>>>      void *nohdr_data;
>>>      struct btf_header hdr;
>>> -     u32 nr_types;
>>> +     u32 nr_types; /* includes VOID for base BTF */
>>>      u32 types_size;
>>>      u32 data_size;
>>>      refcount_t refcnt;
>>>      u32 id;
>>>      struct rcu_head rcu;
>>> +
>>> +     /* split BTF support */
>>> +     struct btf *base_btf;
>>> +     u32 start_id; /* first type ID in this BTF (0 for base BTF) */
>>> +     u32 start_str_off; /* first string offset (0 for base BTF) */
>>> };
>>> 
>>> enum verifier_phase {
>>> @@ -449,14 +454,27 @@ static bool btf_type_is_datasec(const struct btf_type *t)
>>>      return BTF_INFO_KIND(t->info) == BTF_KIND_DATASEC;
>>> }
>>> 
>>> +static u32 btf_nr_types_total(const struct btf *btf)
>>> +{
>>> +     u32 total = 0;
>>> +
>>> +     while (btf) {
>>> +             total += btf->nr_types;
>>> +             btf = btf->base_btf;
>>> +     }
>>> +
>>> +     return total;
>>> +}
>>> +
>>> s32 btf_find_by_name_kind(const struct btf *btf, const char *name, u8 kind)
>>> {
>>>      const struct btf_type *t;
>>>      const char *tname;
>>> -     u32 i;
>>> +     u32 i, total;
>>> 
>>> -     for (i = 1; i <= btf->nr_types; i++) {
>>> -             t = btf->types[i];
>>> +     total = btf_nr_types_total(btf);
>>> +     for (i = 1; i < total; i++) {
>>> +             t = btf_type_by_id(btf, i);
>>>              if (BTF_INFO_KIND(t->info) != kind)
>>>                      continue;
>>> 
>>> @@ -599,8 +617,14 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
>>> 
>>> static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
>>> {
>>> -     return BTF_STR_OFFSET_VALID(offset) &&
>>> -             offset < btf->hdr.str_len;
>>> +     if (!BTF_STR_OFFSET_VALID(offset))
>>> +             return false;
>>> +
>>> +     while (offset < btf->start_str_off)
>>> +             btf = btf->base_btf;
>> 
>> Do we need "if (!btf) return false;" in the while loop? (and some other loops below)
> 
> No, because for base btf start_str_off and start_type_id are always
> zero, so loop condition is always false.

Ah, I misread the code. Thanks for the explanation. 

Acked-by: Song Liu <songliubraving@fb.com>